File::set_times: Add documentation about setting directory timestamps

Inspired by #123883 .

Author: joshtriplett

library/std/src/fs.rs

@@ -1111,6 +1111,10 @@ impl File {
     /// `futimes` on macOS before 10.13) and the `SetFileTime` function on Windows. Note that this
     /// [may change in the future][changes].
     ///
+    /// On most platforms, including UNIX and Windows platforms, this function can also change the
+    /// timestamps of a directory. To do so, open the directory with `File::open`, without
+    /// attempting to obtain write permission.
+    ///
     /// [changes]: io#platform-specific-behavior
     ///
     /// # Errors